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1. Comparison of the Core-collapse Evolution of Two Nearly Equal-mass Progenitors

   Bruenn, S; Severing, A; Lentz, E; Sukhbold, T; His, W; Harris, J; Messer, O; Mezzacappa, A (April 2023, The Astrophysical journal)

   We compare the core-collapse evolution of a pair of 15.8 M☉ stars with significantly different internal structures, a consequence of the bimodal variability exhibited by massive stars during their late evolutionary stages. The 15.78 and 15.79 M☉ progenitors have core masses (masses interior to an entropy of 4 kB baryon−1) of 1.47 and 1.78 M☉ and compactness parameters ξ1.75 of 0.302 and 0.604, respectively. The core-collapse simulations are carried out in 2D to nearly 3 s postbounce and show substantial differences in the times of shock revival and explosion energies. The 15.78 M☉ model begins exploding promptly at 120 ms postbounce when a strong density decrement at the Si– Si/O shell interface, not present in the 15.79 M☉ progenitor, encounters the stalled shock. The 15.79 M☉ model takes 100 ms longer to explode but ultimately produces a more powerful explosion. Both the larger mass accretion rate and the more massive core of the 15.79 M☉ model during the first 0.8 s postbounce time result in larger νe/n ̄e luminosities and RMS energies along with a flatter and higher-density heating region. The more-energetic explosion of the 15.79 M☉ model resulted in the ejection of twice as much 56Ni. Most of the ejecta in both models are moderately proton rich, though counterintuitively the highest electron fraction (Ye = 0.61) ejecta in either model are in the less-energetic 15.78 M☉ model, while the lowest electron fraction (Ye = 0.45) ejecta in either model are in the 15.79 M☉ model. 

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2. Comparison of the Core-collapse Evolution of Two Nearly Equal-mass Progenitors

   Bruenn, S; Sieverding, A; Lentz, E; Sukhbold, T; Hix, W; Huk, L; Harris, J; Messer, O; Mezzacappa, A (April 2023, The Astrophysical journal)

   We compare the core-collapse evolution of a pair of 15.8 M☉ stars with significantly different internal structures, a consequence of the bimodal variability exhibited by massive stars during their late evolutionary stages. The 15.78 and 15.79 M☉ progenitors have core masses (masses interior to an entropy of 4 kB baryon−1) of 1.47 and 1.78 M☉ and compactness parameters ξ1.75 of 0.302 and 0.604, respectively. The core-collapse simulations are carried out in 2D to nearly 3 s postbounce and show substantial differences in the times of shock revival and explosion energies. The 15.78 M☉ model begins exploding promptly at 120 ms postbounce when a strong density decrement at the Si– Si/O shell interface, not present in the 15.79 M☉ progenitor, encounters the stalled shock. The 15.79 M☉ model takes 100 ms longer to explode but ultimately produces a more powerful explosion. Both the larger mass accretion rate and the more massive core of the 15.79 M☉ model during the first 0.8 s postbounce time result in larger νe/n ̄e luminosities and RMS energies along with a flatter and higher-density heating region. The more-energetic explosion of the 15.79 M☉ model resulted in the ejection of twice as much 56Ni. Most of the ejecta in both models are moderately proton rich, though counterintuitively the highest electron fraction (Ye = 0.61) ejecta in either model are in the less-energetic 15.78 M☉ model, while the lowest electron fraction (Ye = 0.45) ejecta in either model are in the 15.79 M☉ model. 

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3. Luminous Radio Emission from the Superluminous Supernova 2017ens at 3.3 yr after Explosion

   https://doi.org/10.3847/2041-8213/acf1fd  

   Margutti, Raffaella; Bright, J S; Matthews, D J; Coppejans, D L; Alexander, K D; Berger, E; Bietenholz, M; Chornock, R; DeMarchi, L; Drout, M R; et al (September 2023, The Astrophysical Journal Letters)

   Abstract We present the results from a multiyear radio campaign of the superluminous supernova (SLSN) SN 2017ens, which yielded the earliest radio detection of an SLSN to date at the age of ∼3.3 yr after explosion. SN 2017ens was not detected at radio frequencies in the first ∼300 days but reachedL_ν≈ 10²⁸erg s⁻¹cm⁻²Hz⁻¹atν∼ 6 GHz, ∼1250 days post explosion. Interpreting the radio observations in the context of synchrotron radiation from the supernova shock interaction with the circumstellar medium (CSM), we infer an effective mass-loss rate $\dot{M}\approx {10}^{-4}{M}_{☉}{\mathrm{yr}}^{-1}$atr∼ 10¹⁷cm from the explosion’s site, for a wind speed ofv_w= 50–60 km s⁻¹as measured from optical spectra. These findings are consistent with the spectroscopic metamorphosis of SN 2017ens from hydrogen poor to hydrogen rich ∼190 days after explosion reported by Chen et al. SN 2017ens is thus an addition to the sample of hydrogen-poor massive progenitors that explode shortly after having lost their hydrogen envelope. The inferred circumstellar densities, implying a CSM mass up to ∼0.5M_☉, and low velocity of the ejection suggest that binary interactions (in the form of common-envelope evolution and subsequent envelope ejection) play a role in shaping the evolution of the stellar progenitors of SLSNe in the ≲500 yr preceding core collapse. 

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4. Three-dimensional General-relativistic Simulations of Neutrino-driven Winds from Rotating Proto-neutron Stars

   https://doi.org/10.3847/1538-4357/ac69da  

   Desai, Dhruv; Siegel, Daniel M.; Metzger, Brian D. (May 2022, The Astrophysical Journal)

   Abstract We explore the effects of rapid rotation on the properties of neutrino-heated winds from proto-neutron stars (PNS) formed in core-collapse supernovae or neutron-star mergers by means of three-dimensional general-relativistic hydrodynamical simulations with M0 neutrino transport. We focus on conditions characteristic of a few seconds into the PNS cooling evolution when the neutrino luminosities obey $L_{{\nu }_e}+L_{{\overline{\nu }}_e}\approx 7\times {10}^{51}$erg s⁻¹, and over which most of the wind mass loss will occur. After an initial transient phase, all of our models reach approximately steady-state outflow solutions with positive energies and sonic surfaces captured on the computational grid. Our nonrotating and slower rotating models (angular velocity relative to Keplerian Ω/Ω_K≲ 0.4; spin periodP≳ 2 ms) generate approximately spherically symmetric outflows with properties in good agreement with previous PNS wind studies. By contrast, our most rapidly spinning PNS solutions (Ω/Ω_K≳ 0.75;P≈ 1 ms) generate outflows focused in the rotational equatorial plane with much higher mass-loss rates (by over an order of magnitude), lower velocities, lower entropy, and lower asymptotic electron fractions, than otherwise similar nonrotating wind solutions. Although such rapidly spinning PNS are likely rare in nature, their atypical nucleosynthetic composition and outsized mass yields could render them important contributors of light neutron-rich nuclei compared to more common slowly rotating PNS birth. Our calculations pave the way to including the combined effects of rotation and a dynamically important large-scale magnetic field on the wind properties within a three-dimensional GRMHD framework. 

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5. Test of light-lepton universality in τ decays with the Belle II experiment

   https://doi.org/10.1007/JHEP08(2024)205  

   Adachi, I; Adamczyk, K; Aggarwal, L; Aihara, H; Akopov, N; Aloisio, A; Anh_Ky, N; Asner, D M; Atmacan, H; Aushev, V; et al (August 2024, Journal of High Energy Physics)

   A<sc>bstract</sc> We present a measurement of the ratio$$ {R}_{\mu }=\mathcal{B}\left({\tau}^{-}\to {\mu}^{-}{\overline{\nu}}_{\mu }{\nu}_{\tau}\right)/\mathcal{B}\left({\tau}^{-}\to {e}^{-}{\overline{\nu}}_e{\nu}_{\tau}\right) $$ $R_{\mu }=B\left({\tau }^{-}\to {\mu }^{-}{\overline{\nu }}_{\mu }{\nu }_{\tau }\right)/B\left({\tau }^{-}\to e^{-}{\overline{\nu }}_e{\nu }_{\tau }\right)$of branching fractions$$ \mathcal{B} $$ $B$of theτlepton decaying to muons or electrons using data collected with the Belle II detector at the SuperKEKBe⁺e⁻collider. The sample has an integrated luminosity of 362 ± 2 fb⁻¹at a centre-of-mass energy of 10.58 GeV. Using an optimised event selection, a binned maximum likelihood fit is performed using the momentum spectra of the electron and muon candidates. The result,R_μ= 0.9675 ± 0.0007 ± 0.0036, where the first uncertainty is statistical and the second is systematic, is the most precise to date. It provides a stringent test of the light-lepton universality, translating to a ratio of the couplings of the muon and electron to theWboson inτdecays of 0.9974 ± 0.0019, in agreement with the standard model expectation of unity. 

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